The present invention relates to power transformer coil-clamping apparatus and particularly to remotely actuated coil-clamping apparatus having a spring-loaded mechanical follow-up device for maintaining the coil clamping force at a predetermined value once the force generated by the coil-clamping apparatus drops below a predetermined value.
It is fairly well known by those skilled in the transformer art that unrestrained transformer windings expand axially when subjected to substantial overload or short-circuit currents. Under such heavy stress conditions, transformer windings telescope with respect to each other. In order to restrain such axial movement, the ends of such windings must be held in a relatively fixed position with respect to each other in order to prevent winding damage and eventual transformer failure.
In order to keep a transformer winding as tight as possible, it is generally preferable to compress the transformer winding to the greatest degree practicable during transformer manufacture. The limits placed upon the amount of compressive force that can be safely used is dependent upon the strength of the winding components as well as the strength of its supporting structure. For example, if winding insulation is overstressed mechanically by excessive compressive force, insulation breakdown and winding failure may result.
At the present time transformer windings are clamped in the factory by a variety of mechanical devices. These devices eventually form a part of a completed transformer. Winding compressing techniques used according to the prior art, have disadvantages which may be detrimental to transformer operation. During normal operation and heat cycling of the transformer winding structure, the amount of compressive force within the winding structure increases and decreases due to winding thermal expansion characteristics. After a sufficient length of time, this cycling results in winding components acquiring a new permanent dimension or set which is normally smaller than the winding dimension at the time of original construction and clamping. After the transformer has been in operation for a period of time, the winding structure tends to loosen with respect to its supporting structure. This just-mentioned loosening reduces the ability of the winding, with its supporting structure, to withstand axial forces resulting from short-circuit or other high current conditions because a loosely mounted transformer winding has the ability to acquire sufficient winding damaging momentum.
One limitation on many of the present clamping systems is that any further clamping of a transformer winding, which has loosened subsequent to completion of manufacture, cannot readily be accomplished without transferring such a transformer to a service shop. Once in a service shop, the previously sealed transformer tank must be opened for access to the mechanism that controls the transformer winding clamping device. Several methods have been suggested in the past to avoid the movement and unsealing of such transformers. These ideas consist mainly of jack screws and other mechanical devices reachable through removable access ports after the transformer is deenergized. Another clamping system is described by G. O. Usry et al. in the IEEE Transactions on Power Apparatus and Systems, July/Aug., Vol. PAS-93 No. 4, at page 1047. In this system a hydraulic clamping system, actuated externally of a transformer enclosure, provides a relatively simple means for adjusting the amount of winding clamping force desired while such a transformer is located in the field. However, a system that hydraulically clamps a transformer winding for an extended period of time has at least one serious shortcoming. Hydraulic pressure associated with a hydraulic clamping device will eventually leak past hydraulic sealing means associated with such a device causing transformer winding clamping force to dissipate.
Other clamping devices for transformer windings that, for some reason or other lose their clamping ability over an extended period of time, place a transformer winding in an equally vulnerable condition, which is that of a transformer winding highly susceptible to permanent damage if subjected to heavy overload currents while in an unclamped state.